1. Field of the Invention
The present invention relates to a screw vacuum pump and, more particularly, to a screw vacuum pump which is designed so that it is possible to reduce the load on the pump at the time of evacuation of a gas at atmospheric pressure.
2. Discussion of the Background
The following are conventional methods of reducing the load at the time of evacuation of a gas at atmospheric pressure in a screw vacuum pump which has a pair of male and female rotors rotating in mesh with each other around two parallel axes, respectively, in a casing:
(1) A method wherein the rotating speed of the screw vacuum pump is lowered at the time of evacuation of a gas under atmospheric pressure, thereby reducing the load on the pump.
(2) A method wherein a valve is provided at the suction side of the screw vacuum pump and the valve is throttled to reduce the load on the pump at the time of evacuation of a gas of atmospheric pressure.
(3) A method wherein the screw vacuum pump is arranged in a two-stage structure comprising a pre-stage pump and a post-stage pump and only the post-stage pump is operated at the time of evacuation of a gas at atmospheric pressure.
The above-described methods (1) to (3) of reducing the load on the pump at the time of evacuation of a gas at of atmospheric pressure suffer from the following disadvantages:
The load reducing method (1) needs an inverter or the like to change the rotating speed of the pump.
The load reducing method (2) necessitates providing a valve at the suction side and also needs a controller for controlling the throttling of the valve.
The load reducing method (3) shortens the lifetime of the machine because the pre-stage pump repeats starting and stopping at the time of evacuation of a gas of atmospheric pressure.